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Soft corals

Soft corals

By Ron Shimek

Soft corals, corals and sea anemones are
categorized together in the Class Anthozoa of the Phylum Cnidaria, and animals in all of
these groups have a lot in common. The basic bottom-dwelling body form in all cnidarians
is that of a polyp, essentially just hollow cylinder with a fringe of

A colony of an octocoral
colony found on one of the ships in Truk Lagoon. The pinnate branches of the tentacles are
quite large and evident.

tentacles around the mouth. All soft
coral bodies are derived from the standard polyp, although in some of them the basic shape
may be very obscure (Kozloff, 1990; Ruppert and Barnes 1994). The differences between the
soft corals as one component of the class Anthozoa, and the stony corals and sea anemones
as another, are profound and have significant repercussions with regard to the biology of
the animals. From the examination of differences in the basic genetic material of these
two groups, it appears that they last shared a common ancestor about 400 million years ago
- plenty of time for major differences to develop. As a comparison, the common ancestors
of both present-day reptiles and mammals lived about half as long ago.

Soft corals,
corals and sea anemones lack the jellyfish or medusa stage characteristic of the other
cnidarians. Additionally, instead of having a simple sac-like body with the mouth as an
opening at the top, their body wall is turned down into the mouth creating a short tube
inside the mouth called the pharynx. Soft corals posses a groove down the length of this
pharynx at one end of the slit-like mouth. This groove, called a siphonoglyph, is lined
with a dense layer of microscopic, hairlike, beating flagella. These flagella continually
pump water into the animal's body; this water current inflates the body, circulates fluids
within the body, and provides a volume of water to act as an antagonist to the action of
the body musculature.

Diagrammatic representation of a soft coral
. The
colony surface is shown on the left, while the internal structure is shown on the right.
The gonads on the septa are shown in red and the siphonoglyph (in the siphonozooid) is
orange. Gastric filaments aid in killing and digesting prey, and fluid circulates through
the interconnected gut canals which are lined with gastrodermis. Spicules are shown in
blue in the yellow mesoglea.

As with the other cnidarians, the soft coral body wall consists of three layers: an
exterior layer of protective epithelial tissue called an epidermis, an interior layer of a
digestive epithelial tissue called a gastrodermis, and a fibrous layer that lies between
these two tissues. This connective fiber layer, the mesoglea or "middle jelly",
generally lacks cells and living components and is composed of proteinaceous collagen
fibers (the same material that constitutes tendons and ligaments in mammals - gristle!).
Other proteins are also found in the mesoglea. Collagen is non-elastic and not extensible
and its presence in the body walls of these animals allows them to resist deformation and
maintain their shape. Many of the soft corals are naturally found in shallow water
environments that have significant wave action and turbulence. The presence of such a
rugged fibrous mesoglea has been shown to be a significant factor in the ability of many
sea anemones to live in wave swept areas, and it is probably as critical with the soft
corals. (Wainwright and Koehl, 1976; Koehl, 1977).

The polyp
portion of the soft-coral gut or gastrovascular cavity is divided by septa - walls of
tissue that connect the pharynx to the outside body wall. There are 8 septa in all soft
corals, and the if the animal were to be cut in cross-section it would look like a wheel
with 8 spokes. The septa are lined with the digestive epithelium and filled with a fibrous
mesogleal core. The sides of each septum have muscle bands on them which run parallel to
the long axis of the polyp. When these muscles contract, the polyp is withdrawn in to the
main body of the colony. The polyps get re-inflated by the water pumped into the animal by
the flagella of the siphonoglyphs.

Taxonomically, the soft corals belong to
the Subclass ALCYONARIA of the Class ANTHOZOA. These animals are also known by the name
Octocorals and the entire group consists of soft corals, sea pens, gorgonians, pipe-organ
coral,
Tubipora
, and the blue coral,
Heliopora
.

A small colony of
Anthelia
from Truk Lagoon. Notice the pinnate tentacles.

As the name octocoral implies, these
animals are characterized by have having only eight tentacles on each polyp.In addition,
these tentacles are pinnate, that is branched in a feather-like pattern, with the branches
arising from opposite sides of the main tentacle branch. The relative prominence of these
pinnate branches varies a lot; in the star polyps,
Pachylavularia
viridis
,
the branches are hardly noticeable, while in some of the
Anthelia
species, they may
be longer than the main branch.

These animals generally have a internal skeleton made
of fused or individual calcium carbonate spicules. In those animals such as
Dendronephthya
klunzingeri
have few widely distributed spicules clearly visible in the body wall.
Dendronephthya
is soft, flexible, and bends in currents, presumably to maximize its suspension feeding
capabilities.

A branch of the Cauliflower coral,
Dendronephthya
klunzingeri
. The scattered white calcareous spicules are quite evident in the
branches

In other species such as the leather
corals,
Sarcophyton
species, the dense arrangement of spicules in the mesoglea
makes for a relatively rigid structure. These colonies are often found in areas with swift
water currents or turbulent wave action. Their collagen and spicular arrangement
facilitates their survival by allowing slow deformation under extreme currents, but
sufficient resistance to the regular currents to allow feeding. Depending on the species,
other minerals may be incorporated into the calcium carbonate matrix, and proteinaceous
material is often utilized as well. The mineral components of the skeleton can color it,
but in most soft corals the spicules are colorless or white.

In addition to an endoskeleton, most soft corals have quite a complicated apparatus of
internal plumbing which serves to distribute food and nutrients throughout the colony. All
of the guts of all of the polyps connect to this conductive system, and so food can get
distributed throughout the entire colony. Obviously, as well, wastes will also be
dispersed either out of the colony with excess water, or near enough to the tissue surface
so that the material will diffuse out.

For more information on soft coral morphology and biology follow this link, but note
that they mistakenly use the term "pennate" (an adjective describing diatom
shapes) for "pinnate," the adjective describing octocoral tentacle shapes.
Intro to soft corals -
pennate

Soft corals are considered to be colonial animals, where the main body of the animal is
often derived from a single polyp and with other polyps differentiated to do other
functions. Generally the first polyp that forms after the metamorphosis of the larva forms
the basic polyp, and other polypoid individuals are budded off of it during growth. The
shapes of these polyps can change both with growth and aging of the colony.

Some of the soft corals and sea pens are the most
polymorphic of the anthozoans. Typically the leather corals, in the Order Alcyonacea, have
several types of polyps. There are tentaculate, feeding individuals (gastrozooids)
arranged on the upper surface. These polyps are retractile, and when fully withdrawn, the
surface of the colony appears smooth. When the polyps are extended, they may be seen to
have eight small tentacles, although in some species the tentacles are represented only by
eight small bumps. Often over the colony upper surface are small zooids which lack
tentacles. These zooids (siphonozooids) are basically just a mouth with a siphonoglyph in
it and function to inflate the colony by pumping water into the common gastrovascular
cavity. When the feeding individuals are extended, the siphonozooids appear as small dots
between them.

Most tropical soft corals are actively feeding animals(Leversee,
1976; Lasker, 1981; Lasker, et al. 1983; Patterson, 1984, 1991; Sorokin, 1990a,b), but
many get supplemental nutrition from zooxanthellae (Dai and Lin, 1993; Dauget, 1992). This
facilitates their ease of maintenance, and many species may be kept without too many
problems in marine aquaria. Although many species appear to eat many different types of
particulate food, some such as the cauliflower corals,
Dendronephthya
, appear to be
mostly herbivorous eating only phytoplankton. As phytoplankton are generally lacking in
captive systems, and adding sufficient phytoplankton from culture is either difficult or
expensive, captive husbandry of these animals should not be attempted by most hobbyists
(Fabricius, et al. 1995) (See the article in the December, 1996 issue of Aquarium Net,
Dendronephthya
A Seduction of
Allusions and Illusions by Eric Borneman
).

A few species, such as some
Xenia
may have a reduced gut,
and not feed; these animals do well under reasonably strong light, although extreme
intensities are unnecessary. I have a colony that is growing very well under power compact
fluorescent lights and it is subdividing into new colonies at the rate of about one to
three new colonies per week. Others, such as sea pens, are suspension-feeding animals that
appear to grow in a manner to maximize their water filtering efficiency; zooxanthellae
appear to supplement their diet. They feed on small zooplankton which they catch and
subdue utilizing their nematocysts.

Soft corals, particularly the leather
corals and gorgonians, may be aggressive to their neighbor and in these situations they
often use chemical warfare (Sammarco, et al. 1983; Sammarco, et al. 1985). It has recently
been demonstrated that many leather corals and other soft corals produce complex organic
chemicals that are poisonous to stony corals (Aceret, et al. 1995).

They often produce small amounts of these
chemicals, but their chemical production accelerates when they are in the presence of
stony corals(Pawlik and Fenical, 1992). Such chemicals may accumulate in reef aquaria and
affect other animals. Additionally, many of them seem to secrete antifouling agents,
either to prevent diatom and bacterial growth or to inhibit settling by other marine
organisms (Kim 1994). Although soft corals, leather corals, gorgonians and many other
alcyonarians, are permanently attached to hard substrates, a few others primarily sea pens
and their relatives are found in unconsolidated sediments and many of them are quite
mobile. They may move up and down in the sediments many times each day or remain immobile
for long periods of time and then become quite mobile for a short time. Some of them, such
as the Northeastern Pacific sea pen,
Ptilosarcus
gurneyi,
have been observed
to "fly" or "balloon" through the water over the substrate. This
species and some others appear to be able to generate hydrodynamic lift when water flows
over its "leaves". Even the more sessile ones may move, however, by differential
growth and probably some sort of slow creeping(La Barre, and Coll. 1982). Colonies of
Xenia
are quite mobile, albeit on a slow time scale; mine seem to be able to move at the rate of
about a centimeter a week.

As with all animals that are basically sessile, soft corals can be
considered to mounds of flesh waiting to become somebody’s meal (Griffith, 1994).
There are many predators upon them. The best studied examples of soft coral predation are
the sea pen beds of the North Eastern Pacific and some gorgonians in the Caribbean
(Harvell, et al. 1993). Sea pens are soft-sediment dwelling soft corals that are often
found in great beds. In these situations, they act as a food resource for many different
types of predators. In subtidal areas around Seattle, Washington, the sea pens are preyed
upon by five species of nudibranchs and four different sea stars (Birkeland, 1974).

Some Caribbean gorgonians are
preyed upon by the Flamingo Tongue snails,
Cyphoma
gibbosum
, and one species
of fireworm, while others are eaten by file fish (Vreeland, and Lasker. 1989). Often these
soft corals have chemical defenses against some predators (Coll, et al. 1982; Targett, et
al. 1983; Van-Alstyne, and Paul. 1992; Harvell, et al. 1993).

A Caribbean gorgonian showing the snails,
Cyphoma
gibbosum
,
eating the flesh of the branches and a file fish which was plucking polyps off the
branches.

Leather corals seem to be eaten by a number of
different animals, but probably the most insidious from the aspect of the marine aquarist
is the snail,
Rapa
rapa
(Kohn, 1983). These snails can bore there way into a
leather coral leaving only a small hole to mark their presence. Once inside they have the
potential of completely eating the whole leather coral, but do it from the inside. The
leather coral will appear to be fine, until one day it starts to disintegrate. If the
hobbyist examine the coral basically all they find is a hollow shell and a rather large,
about walnut-sized, snail in the cavity. In these cases, the colony may regenerate from
the tissue fragments, but whole colony is no longer viable.

These potential predators
aside, many soft-corals are amongst the easiest of animals for reef-aquarists to maintain.
Coupled with their pleasing shapes and often striking colors, they are a good choice for
marine aquaria. Their colonial morphology facilitates asexual reproduction, and many of
the animals actively subdivide and reproduce in our systems. Others may be purposely split
to provide many daughter colonies. They are good animals for both beginning and advanced
reef aquarists.